The photoresponsive elements of the best known commercial solar cells are monocrystalline silicon and a junction of polycrystalline cadmium sulfide and copper sulfide (Clevite cell). The solar cells comprising monocrystalline silicon wafers can convert solar energy to electrical power with a degree of efficiency of up to 15% and are presently used in satellites and for some special uses on earth. Solar cells comprising monocrystalline semiconductive layers are considered too expensive for use as current producers in competition with conventional power sources.
The solar cells of polycrystalline cadmium sulfide and copper sulfide are substantially cheaper to produce than monocrystalline silicon wafers, but unfortunately, they have a level of efficiency of only 5 to 6%. Besides this relatively low level of efficiency, their technical use on a wide scale is also hindered by high production costs and short operating life or stability with regard to their electrical properties.
Elements of the semiconducting compositions of the present invention have been suggested in various combinations in the prior art. For example, U.S. Pat. No. 3,104,188 discloses a solar cell junction between cadmium selenide and a suitable metal, with disclosed suitable metals being nickel, chromium, platinum, gold, and silver. A tin electrode may be used. U.S. Pat. No. 2,276,198 concerns a photocell which is a selenium semiconductor upon a base electrode plate and having thereon thin films of a fourth group metal, such as tin, and then a platinum or silver film. U.S. Pat. No. 3,226,271 and 3,342,546 include teachings of possible interest as to CdSe with the first disclosing tin useful to dope the CdSe and with the second including the phase diagram of cadmium and selenium. A solar photovoltaic cell including CdSe in a graded energy gap is taught in U.S. Pat. No. 3,496,024. Example 13 in U.S. Pat. No. 3,811,953 speaks of using CdS/cadmium stannate (Cd.sub.2 SnO.sub.4) for generation of electrical potential by sunlight exposure.